1. Field of the Invention
The present invention relates to a motor vehicle steering system which is adapted to turn steerable vehicle wheels on the basis of the operation of an operation member such as a steering wheel.
2. Description of Related Art
A steering system for a motor vehicle (so-called steer-by-wire system) has been proposed, which has no mechanical coupling between a steering wheel and a steering mechanism for turning steerable vehicle wheels, and is adapted to detect the direction and degree of the operation of the steering wheel and apply a driving force from a steering actuator such as an electric motor to the steering mechanism on the basis of the result of the detection.
This arrangement makes it possible to flexibly change the ratio (gear ratio) of the degree of the rotation of the steering wheel to the degree of the turning of the steerable vehicle wheels depending on the traveling state of the vehicle for improvement of the maneuverability of the motor vehicle. The aforesaid arrangement advantageously prevents upward thrust of the steering wheel in the event of the collision of the motor vehicle, and allows for more flexible placement of the steering wheel.
Since the steer-by-wire system has no mechanical coupling between the steering wheel and the steering mechanism, it is important to give a fail-safe consideration to a steering drive system. To this end, it is proposed to provide a clutch between the steering wheel and the steering mechanism for engagement and disengagement therebetween, whereby a mechanical coupling is established between the steering wheel and the steering mechanism with the clutch brought into engagement in the event of malfunction of the system.
With the clutch interposed between the steering wheel and the steering mechanism, however, there is a possibility that the turning of the steerable vehicle wheels by the steering actuator undesirably causes passive rotation of the steering wheel when the drive control of the steering actuator is continued after the engagement of the clutch. Therefore, the driving of the steering actuator is stopped even if any component other than the steering actuator malfunctions. As a result, the steering wheel feels very heavy after the engagement of the clutch, so that a greater load is exerted on the clutch.
It is an object of the present invention to provide a motor vehicle steering system which is capable of properly performing a steering operation even if a malfunction occurs.
The motor vehicle steering system according to the present invention comprises: an operation member for steering a motor vehicle; a steering mechanism for turning steerable wheels of the motor vehicle; a first actuator and a second actuator each operative to apply a driving force to the steering mechanism; and a steering control section which, when the first and second actuators operate normally, causes at least one of the first and second actuators to apply the driving force to the steering mechanism and, when one of the first and second actuators malfunctions, causes the other normal actuator to apply the driving force to the steering mechanism.
In accordance with a first aspect of the present invention, more specifically, there is provided a motor vehicle steering system, which comprises: an operation member for steering a motor vehicle; a steering mechanism for turning steerable wheels of the motor vehicle; a primary steering actuator and a secondary steering actuator coupled to the steering mechanism; a clutch mechanism operative to mechanically engage and disengage the operation member with respect to the steering mechanism; and a steering control section which, when the primary steering actuator and the secondary steering actuator operate normally, brings the clutch mechanism into a disengaged state to cause the primary steering actuator and the secondary steering actuator to generate a driving force for driving the steering mechanism and, when one of the primary steering actuator and the secondary steering actuator malfunctions, brings the clutch mechanism into an engaged state to cause the other normal steering actuator to generate a steering assist force according to operation of the operation member.
According to this arrangement, when the primary steering actuator and the secondary steering actuator operate normally, the driving force is applied to the steering mechanism from both of the primary steering actuator and the secondary steering actuator. Even if the primary steering actuator and the secondary steering actuator each have a relatively small maximum output, the driving force can sufficiently be applied to the steering mechanism to properly perform a steering operation.
If one of the primary steering actuator and the secondary steering actuator malfunctions, the clutch mechanism is brought into the engaged state, and the other normal steering actuator is controlled as a source of the steering assist force. Thus, the normal steering actuator applies the steering assist force to the steering mechanism according to the operation of the operation member. Therefore, the operation member can easily be operated even after the engagement of the clutch, allowing for a practical fail-safe steering operation.
The motor vehicle steering system may further comprise a counter-force actuator for applying an operation reaction force to the operation member. In this case, the steering control section is preferably adapted to cause the counter-force actuator to generate an additional steering assist force according to the operation of the operation member when one of the primary steering actuator and the secondary steering actuator malfunctions.
With this arrangement, the counter-force actuator is also controlled as the source of the steering assist force if the primary steering actuator or the secondary steering actuator malfunctions. Thus, a further greater steering assist force can be applied to the steering mechanism, so that the operation member can more easily be operated after the engagement of the clutch mechanism.
Where the motor vehicle steering system further comprises the counter-force actuator for applying the operation reaction force to the operation member, the steering control section is adapted to bring the clutch mechanism into the engaged state to cause the primary steering actuator and the secondary steering actuator to generate the steering assist force according to the operation of the operation member when the counter-force actuator malfunctions.
With this arrangement, the clutch mechanism is brought into the engaged state, and the primary steering actuator and the secondary steering actuator are controlled as the source of the steering assist force if the counter-force actuator malfunctions. Therefore, the operation member (e.g., steering wheel) can easily be operated, allowing for a practical fail-safe steering operation.
In accordance with a second aspect of the present invention, there is provided a motor vehicle steering system, which comprises: an operation member for steering a motor vehicle; a steering mechanism for turning steerable wheels of the motor vehicle; a primary steering actuator and a secondary steering actuator coupled to the steering mechanism and having substantially the same maximum output; and steering control means which, when the primary steering actuator and the secondary steering actuator operate normally, causes the primary steering actuator and the secondary steering actuator to generate a driving force for driving the steering mechanism and, when one of the primary steering actuator and the secondary steering actuator malfunctions, causes the other normal steering actuator to generate the driving force for driving the steering mechanism.
According to the present invention, when the primary steering actuator and the secondary steering actuator operate normally, the driving force is applied to the steering mechanism from both of the primary steering actuator and the secondary steering actuator. Therefore, the maximum outputs of the primary steering actuator and the secondary steering actuator can each be set at a smaller level.
The primary steering actuator and the secondary steering actuator preferably each have a maximum output which is 60% to 70% of an output sufficient to drive the steering mechanism. In this case, the driving force can sufficiently be applied to the steering mechanism to properly perform the steering operation, when the primary steering actuator and the secondary steering actuator operate normally. Even if only one of the primary steering actuator and the secondary steering actuator operates normally, a driving force sufficient to maintain the direction of the steerable vehicle wheels can be applied to the steering mechanism.
In accordance with a third aspect of the present invention, there is provided a motor vehicle steering system, which comprises: an operation member for steering a motor vehicle; a steering mechanism for turning steerable wheels of the motor vehicle; a steering actuator coupled to the steering mechanism; a clutch mechanism operative to mechanically engage and disengage the operation member with respect to the steering mechanism; a counter-force actuator for applying an operation reaction force to the operation member; and a steering control section which, when the steering actuator operates normally, brings the clutch mechanism into a disengaged state to cause the steering actuator to generate a driving force for driving the steering mechanism and, when the steering actuator malfunctions, brings the clutch mechanism into an engaged state to cause the counter-force actuator to generate a steering assist force according to operation of the operation member.
According to the present invention, the clutch mechanism is brought into the engaged state and the counter-force actuator is controlled as a source of the steering assist force, if the steering actuator malfunctions. Thus, the counter-force actuator applies the steering assist force to the steering mechanism according to the operation of the operation member. Therefore, the operation member can easily be operated after the engagement of the clutch mechanism, allowing for a practical fail-safe steering operation.
The foregoing and other objects, features and effects of the present invention will become more apparent from the following description of the preferred embodiments with reference to the attached drawings.